The present invention relates to hydrostatic transaxles intended primarily for use in the lawn and garden industry on riding lawnmowers, lawn and garden tractors and the like, but may also be applied to larger implements and vehicles.
Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, typically oil, and then back to rotary mechanical motion to rotate a pair of drive axles in order to drive the vehicle. The hydrostatic transmission controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. Such transmissions have utilized radial piston pumps and motors, axial piston pumps and motors and hybrid transmissions wherein the pump may be of the radial piston design, for example, and motor formed as a gear pump. The speed of the output of the transmission is typically controlled by varying the eccentricity of the pump track ring or swash plate.
In recent years, and particularly for smaller displacement applications, it has been common practice to integrate the hydrostatic transmission within the axle casing that also contains the differential and bearings for the two axles. Such casings are typically split along a horizontal plane containing the axis of the axles, and the casing itself is formed of only two parts. However, it is also known to utilize casings comprising three or more components wherein the casing components are attached to each other along vertical split lines or horizontal and vertical split lines.
Although utilizing a single casing for both the transmission and axle gear components necessitates only two large die castings, there are certain disadvantages inherent in such a design. One such disadvantage is that servicing of the hydrostatic transmission or the gear train/differential requires that the entire casing be opened, the oil drained and the complete mechanism withdrawn in order to perform such service. Furthermore, units wherein the hydrostatic transmission is sized for different displacements, for example, use with larger lawn and garden tractors, will require completely different transaxle casings. Since such casings are quite large and require expensive tooling to manufacture, this represents a significant additional cost. Furthermore, integrated hydrostatic transaxles are often noisy, especially when accelerating.
A typical construction of the hydrostatic transmission component of the transaxle includes a one-piece block common to both the pump and motor units, often referred to as a “center section.” The center section facilitates an external mounting surface for a motor barrel and a pump barrel, and additionally, an internal valve body for providing hydraulic communication between the pump and motor barrels. Conventionally, the pump and motor barrel axes of rotation are 90 degrees to one another. Center section machining is difficult because the center section is substantial in size and machined surfaces are substantially perpendicular, often requiring multi-axis machining capabilities. This corresponds to a significant cost associated with this design type.
Moreover, many HSTs heretofore require that the pump and motor mechanism unit be matched to a fixed swash plate prior to mounting the mechanism into the casing. Typically, assembly requires positioning the fixed swash plate in the casing, mounting the pump and motor mechanism into the casing then taking measures to ensure the fixed swash plate is suitably aligned with the pump and motor mechanism. Arranging the fixed swash plate and pump and motor mechanism, in the manner described above, poses a significant step in the assembly process which represents additional cost.